The present invention relates to a method for computing vehicle speed from a signal provided by a vehicle speed sensor. More particularly, the invention presents such a method which provides a stabilized or optimized vehicle speed value for use by vehicle speed control systems.
Many modern vehicles include a variety of electronic controls, particularly an electronic engine control system for the internal combustion engine. Most such electronic engine control systems provide cruise control or vehicle speed governing capabilities. Such systems and algorithms for cruise control or closed loop vehicle speed are well known in the art.
All such systems require as input some value indicative of the actual vehicle road speed which is derived from a speed sensor. Many types of sensors are available for generating a signal indicative of the vehicle speed, most of which are of the pulse generator type. One such pulse generator is a variable reluctance device in which a magnetic pickup located adjacent a rotating component of the drive train produces a pulse with every rotation of the drive train component. In another similar sensor, known as the MINIGEN.TM. sold by Synchrastart Co., a mechanical cable is driven by the drive train and the cable rotation is sensed. The timing of the sensor pulses is typically determined by counting the time for a given number of pulses to pass. This timing value is then used to calculate the actual vehicle speed by application of a number of conversion factors.
One difficulty with speed sensors of this type is that the pulse signals have a wide output tolerance and are often subject to errors and inaccuracies. Such errors include a wide variation in time between pulses which can lead to erroneous vehicle speed calculations. Most cruise control systems require a relatively higher accuracy vehicle speed value. Inaccurate or widely varying vehicle speed values can cause a cruise control to oscillate about the vehicle speed set point as it governs the vehicle speed. Some sensors are available that can generate a clean narrow tolerance pulse train, but these sensors are more expensive than the less accurate V/R type sensors. Moreover, many of the more sophisticated sensors are not readily adapted to the harsh environment of the vehicle drive train as the V/R speed pickup sensors. Vibration, dirt and electrical or magnetic interference can render the more sophisticated sensor impractical for use in an automotive setting.
There is a need for a method for processing the pulse train from a relatively low accuracy vehicle speed sensor and generating a higher quality accurate vehicle speed value for use by the cruise control or vehicle speed governing routines of an electronic engine control.